A ROLE FOR SUCROSE SYNTHASE IN STARCH SYNTHESIS IN SWEET POTATO

在植物中，蔗糖是一种能量和碳水化合物来源。蔗糖是在光合作用的组织中合成的，并被运输到其他植物器官，在那里它被裂解成单糖或衍生物后使用。蔗糖合成酶(SUS)是参与蔗糖分解的酶之一。为了了解甘薯糖分配和代谢的分子机制，我们克隆了编码甘薯SUS的1,191个碱基的基因(IBSUS)。IBSUS推导的氨基酸序列与马铃薯SUS的同源性最高(91%)，与单子叶序列(水稻和玉米)的同源性较低(75%)。已发表的拟南芥SUS序列出人意料地显示出更低的同源性，仅为69%。甘薯根系中的SUS活性在块根发育过程中呈上升趋势。酶活性的提高伴随着淀粉含量的增加。块根中的SUS活性高于叶片、叶柄、茎和须根。在叶片中未检测到酶活性。利用Normal杂交技术研究了编码SUS基因在甘薯不同组织中的表达。SUS基因在叶柄、茎、须根和块根中表达，大小约为2.4kBP。该基因在块根中的表达最为丰富。在叶柄和茎中表达水平较低，而在叶片中未检测到表达。编码SUS的基因在RNA水平上受蔗糖强烈诱导。

In plants, sucrose is an energy and carbohydrate source. Sucrose is synthesized in photosynthetically sctive tissues and transported to other plant organs where it is used after cleavage to monosaccharides or derivatives. Sucrose synthase (SUS) is one of the enzymes involved in sucrose breakdown. To understand the molecular mechanisms in sugar distribution and metabolism of sweet potato, we have isolated a cDNA (1,191 bp) encoding SUS of sweet potato (IBSUS). The deduced amino acid of IBSUS was most closely related (91% homology) to potato SUS.Less homology (75%) was found to monocotyledonous sequences (rice and maize). The published Arabidopsis SUS sequence unexpectedly showed an even lower homology value of 69%. The activity of SUS in roots of sweet potato increased during development of tuberous roots. The enhancement of the enzymatic activity was accompanied by an increase in starch content. The activity of SUS in the tuberous roots was higher than that in the other tissues, namely, leaf blades, petioles, stems and fibrous roots. The enzymatic activity was not detectable in the leaf blade. Expression of the gene encoding SUS in different tissues of sweet potato was assessed by northem bolt hybridization. A signal of the SUS mRNA with a size of approximately 2.4 kbp was found in petioles, stems, fibrous roots and tuberous roots. The gene was most abundantly expressed in tuberous roots. Lower expression levels were found in petioles and stems, whereas no expression could be detected in leaf blades. The gene encoding SUS was strongly inducible by sucrose at the RNA level.